1. Field of the Invention
This invention relates to method for alkylating an alkylatable isoparaffin with an alkylating agent in the presence of an acid alkylation catalyst. More particularly, this invention relates to a method for alkylating an alkylatable isoparaffin with an alkylating agent selected from the class consisting of an olefin and an alkyl sulfate ester in the presence of a mixture of hydrofluoric acid and sulfuric acid as the alkylation catalyst.
2. Description of the Prior Art
In the petroleum industry, alkylation refers to the catalytic reaction of isoparaffins with light olefins, such as ethylene, propylene, butylenes, and amylenes, to produce highly branched paraffins, the alkylate. These branched paraffins are of high octane quality and have relatively low volatility. They are therefore desirable blending components for gasoline.
The concern over air pollution from automobile exhausts has stimulated interest in alkylation. Specifically, legislation in the United States has restricted the use of some ingredients in motor fuels. Limitations are being placed on gasoline additives, such as lead-containing compounds, and limits on the aromatics content have been proposed. Alkylate is not only a highly satisfactory blend stock for present-day gasolines containing lead, but is also desirable in low-lead or no-lead blends. Thus, alkylation may become even more important to the refining industry as a means of producing gasoline with satisfactory antiknock and volatility characteristics.
In particular, isobutane-olefin alkylation, catalyzed by sulfuric acid or hydrofluoric acid, is an important and growing refinery process. U.S. alkylation capacity has approximately doubled in the past 10 years and has now reached over 850,000 barrels per day. Despite the importance and extensive use of alkylation, two areas still remain where substantial improvements can be made: octane number is below potential, and catalyst consumption is high.
Octane numbers could be increased if the alkylation reaction could be made more selective. The ideal isobutane-olefin alkylation reaction is one wherein isobutane adds to a butene to give trimethylpentanes, which have, in general, the highest octane numbers. Reaction conditions are set to produce this high octane number product by maintaining high isobutane to olefin concentrations to minimize olefin-olefin reactions. Yet, in spite of this, about 40% of the olefin reactant polymerizes to a C.sub.12 species, which reacts further to give a mixture of hydrocarbons from C.sub.5 to C.sub.13, having octane numbers ranging from 60-90. Therefore, suppression of the polymerization-related reactions would greatly improve gasoline quality.
The formation of by-products also results in high catalyst consumption. Acid-soluble impurities, such as complex polyolefinic hydrocarbons, and oxidative by-products, such as water and sulfonic acids, are the major undesirable by-products. These products dissolve in and react with the acid catalyst so that the spent catalyst has to be withdrawn and replaced by fresh acid continuously. The costs of replacing catalysts are substantial. Any lengthening of catalyst life would reduce costs considerably.
To this end, numerous alkylation catalyst systems have been investigated. Much research activity has been directed towards the use of additives and promoters in catalyst systems to increase yields, improve quality, and reduce acid consumption. For example, the catalyst systems disclosed in U.S. Pat. Nos. 2,427,293; 2,259,723; 2,387,162; 2,441,102; 2,441,103; 2,545,875; 2,591,367; 2,701,184; 3,187,066; 3,221,071; and 3,766,293 are illustrative of catalyst systems for use in the alkylation of isoparaffins with olefins to produce highly branched isoparaffins. Matuszak, U.S. Pat. No. 2,427,293 (1947) discloses a process for first alkylating excess isobutane with isobutylene in the presence of concentrated hydrofluoric acid, separating the resulting hydrocarbon phase from the hydrofluoric acid phase, and alkylating the remaining unreacted isobutane in the separated hydrocarbon phase with normal butene in the presence of concentrated sulfuric acid. This patent discloses the use of both sulfuric acid and hydrofluoric acid as catalyst in an alkylation process but these two catalysts are taught to be used sequentially in a two-step alkylation, not simultaneously in a one-step alkylation. Further, each step in the process of U.S. Pat. No. 2,427,293 involves a different alkylating agent.
On the other hand, the catalyst systems disclosed in U.S. Pat. Nos. 2,259,723; 2,387,162; 2,441,102; 2,441,103; 2,545,875; 2,591,367; 2,701,184; 3,187,066; 3,221,071; and 3,766,293 are illustrative of one-step processes for alkylating an isoparaffin with an olefin. Meadow, U.S. Pat. Nos. 2,441,102 (1948) and 2,441,103 (1948) states in discussions of the prior art that it is known to effect the alkylation of paraffinic hydrocarbons with olefinic hydrocarbons in the presence of alkylation catalysts consisting of mixtures of sulfuric acid and hydrofluoric acid. Cade, U.S. Pat. No. 2,545,875 (1951) discloses alkylation catalysts comprising concentrated hydrofluoric acid, concentrated sulfuric acid, concentrated phosphoric acid, halosulfonic acids, mixtures of the acids mentioned, and aluminum halide-hydrocarbon complexes. These catalysts are sometimes promoted with such materials as hydrogen halides, free halogens, and boron trifluoride. McAllester, U.S. Pat. No. 2,591,367 (1952) discloses alkylation catalysts comprising concentrated sulfuric acid, anhydrous or aqueous hydrogen fluoride, phosphoric acid, chlorosulfuric acid or fluorosulfuric acid, hydrofluoroboric acid, etc. The patent states that the catalyst system may be such individual catalyst acids, mixtures of such acids, or mixtures of one or more such catalysts with other materials having a beneficial effect on the reaction, for example, boron fluoride, phosphorus pentoxide, oxides of vanadium, zinc or cadmium phosphate, sulfur dioxide and/or trioxide. Rupp, U.S. Pat. No. 2,701,184 (1955) contains disclosure of an alkylation catalyst as being any suitable catalyst material in addition to sulfuric acid, such as mixtures of sulfuric acid and phosphoric acid, hydrofluoric acid and certain complexes of aluminum chloride and boron fluoride. Nathan, U.S. Pat. No. 3,187,066 (1965) discloses alkylation catalysts which include mineral acids, such as sulfuric acid, hydrofluoric acid, phosphoric acid, chlorosulfuric acid, fluorosulfuric acid, etc., which may be used either singly or in mixtures. Stahley, U.S. Pat. No. 3,221,071 (1965) discloses alkylation catalysts including at least one Friedel-Crafts or Lewis acid catalyst or such catalysts with promoters and states that such Lewis acids include hydrofluoric acid, sulfuric acid, phosphorus pentoxide-sulfuric acid mixtures, orthophosphoric acid, pyrophosphoric acid, and the like. However, the relevant disclosures of these patents were limited to the brief remarks stated hereinabove.
On the other hand, U.S. Pat. Nos. 2,259,723; 2,387,162; and 3,766,292 contain broader disclosures. Ballard et al., U.S. Pat. No. 2,259,723 (1941) discloses an alkylation catalyst comprising a mixture of sulfuric acid, halo-sulfonic acid, or mixtures thereof, and a hydrogen halide, specifically, a hydrogen halide of the class consisting of hydrogen bromide and hydrogen chloride. Matuszak, U.S. Pat. No. 2,387,162 (1945) discloses an alkylation catalyst comprising concentrated hydrofluoric acid, concentrated sulfuric acid, mixtures of sulfuric and hydrofluoric acids, fluorosulfuric acid, chlorosulfuric acid, concentrated hydrofluoric acid containing small proportions of boron trifluoride, boron trifluoride-water complexes, phosphoric acid containing dissolved boron trifluoride, aluminum chloride-hydrocarbon complexes, and the like. Parker et al., U.S. Pat. No. 3,766,293 (1973) discloses an alkylation catalyst comprising a major amount of a strong acid catalyst comprising fluorosulfuric acid and a minor amount of one or more catalyst promoters comprising inorganic acids like hydrofluoric acid or sulfuric acid.
Thus far, no one has disclosed the specific components and the specific proportions thereof that are used in the catalyst system of this invention to thereby produce the unexpected advantages of inhibiting undesirable side reactions, increasing catalyst lifetime, producing the maximum yield of desirable products, and producing the maximum octane number alkylate.